Locusts

Locust plagues have been recorded since very early in human history. In the Bible, there is reference to events which happened thousands years ago, viz. locust swarming as one of the ten plagues of Egypt. A locust plague is due to a big swarm of short antennae grasshoppers, called locusts, moving a long distance through several countries, denuding all vegetation of leaves and tender shoots in their way, and bringing in their wake famine, hunger and death in the countries covered. Such locust attacks may show an irregular periodicity, that is appearing every three, four or more years.

The grasshoppers around us may be minor or somewhat serious pests, but they do not form such devastating swarms. How do these grasshoppers differ from locusts? A major breakthrough in solving this problem was made by Uvarov (1928) and Uvarov and Zolotarevsky (1929). They gave what is known as the Phase Theory of Locusts. According to this theory, locusts are highly polymorphic (that is exhibiting two or more forms) grasshoppers. They exhibit two strikingly different forms or phases, a solitary or nonswarming form (phasis solitaria) and a gregarious or swarming phase (phasis gregaria). In addition, there are intermediate or transitory phases (phasis transiens). In the transitory phases are several intermediate forms, showing gradual changes from one extreme form to the other, i.e. from the solitary to the gregarious phase and vice versa. Locusts occur in three subfamilies of the family Acridiidae, Cyrtacanthacridinae, Oedipodinae and Gomphocerinae.

Locust swarms are formed in limited vegetation covered areas surrounded by arid stretches of land. Oases in deserts are favourable spots for rearing of swarms. In such a spot solitary locusts go on feeding and multiplying for some time. They quietly eat away the green food available in abundance. As their population density increases, individuals gradually acquire darker skin pigmentation, change in proportions of certain body parts, and tend to become more active; thus, they change in the direction of the swarming phase. Then in a certain climatically favourable year their population size increases so much that typical migratory phase adults are formed. Such migratory phase locusts look so different from solitary phase individuals that formerly they were regarded as distinct species or genus (e.g. Pachytylus). A situation, which may promote swarm formation in locusts: concentration, aggregation and gregarization are so often dependant of a drying out of the habitat, following good breeding conditions (Chapman, in Resh and Carde, 2003).

The migratory adults leave as a swarm, and this results in outbreak of a locust plague. Locusts are mostly daytime flyers, and the swarm has no directional movement and is carried downwind. Even if flights are generally downwind, there is, however evidence that the insects can maintain sometimes a particular direction. Night flights are known in the Australian plague locust, Chorthoicetes terminifera. In 1988, the swarms, coming from Africa, were carried with the wind to the Caribbean and the northern coast of South America, around 6000 km from their origin in West Africa. PJ still remembers the huge swarms passing through Ethiopia in 1961, devastating everything on their passage. He was once obliged to stop his land-rover since the sky was entirely black with insects.

When the swarm has departed, a number of individuals, which have still only partly changed in the direction of the migratory phase, remain in the habitat of swarm formation. Generation after generation they gradually change to the solitary phase. Population build up continues beyond this phase, and, when the population density has reached a certain incipient value, transition in the direction of the migratory phase again starts, and the cycle is repeated.

Typical solitary phase individuals are leafy green and not very active. But typical migratory phase adults have a pattern of orange and black pigmentation patches on their body, are very active, and have relatively longer femora of the third legs and longer fore wings or tegmina.

What is the mechanism of phase transformation? The problem remains to be solved in satisfying details. But some clues could be deduced from the studies made by a number of workers across the globe. It has been inferred that crowding leads to visual and tactile stimuli. How these stimuli, received by the nervous system, affect the effector organs is not very clear, but neuroendocrine integration (i.e. coordination between the nervous system and the system of hormone producing or endocrine glands) has an important role to play in this. Among the endocrine centres the glands, called corpora allata (singular: corpus allatum, abbr. CA) have a definite role in phase determination. These glands are small ovoid bodies, located in the head beneath the brain. They are connected by nerves with another pair of endocrine centers, corpora cardiaca (singular: corpus cardiacum, abbr. CC), situated a little higher up, closer to the undersurface of the brain. The CC are in turn connected to the middle part of the brain by nerves.

Joly (1955) was first to get definite indications of involvement of CA in phase determination in the locust (Locusta). He found that, on implantation ventral corpus nerve cord ventral corpus nerve cord

suboesophageal ganglion

— Fig. 40.1. A vertical longitudinal cut through the head of Locusta (after Staal, 1961).

of additional CA from another hopper, a green pigment appeared in the blood of the recepient nymph, and, after the next moult, the pigment appeared in its integument. After some more moults completely green hoppers resulted even in crowded conditions. It may be recalled here that hoppers in the solitary phase have leafy green body colour. These observations have been repeated and confirmed by Staal (1961). In addition, Staal has found that, if CA are removed by microsurgery in the 4th nymph of Locusta, the next nymphal stage, i.e. the 5th nymph shows a pattern of yellow and dark pigmentation in the integument, which is characteristic of the gregarious phase, and shows almost adult like features. Staal has also found that, if CA are removed in the 4th nymph, in the 5th nymph the ratio F/C became reduced as compared to what it should be in a solitary phase 5th nymph. (F= length of femur of the third leg; C= width of the head where it the broadest). It may be pointed out here that the ratio F/C has a lower value in gregarious phase nymphs than in solitary phase ones at the same stage. Thus, it has been inferred that deficiency of the CA hormone is a necessary factor in the development of the gregarious phase.

Some other facts, which probably have relevance with the formation of the migratory phase in locusts: A peptide hormone induces the dark coloration of gregarious nymphs. Pheromones (phenylacetonitriles) accelerate maturation, and others are also involved in the maintenance of gregarization. Gregarious coloration has probably an aposematic value. Lizards avoid to eat insects, which have fed on toxic plants.

It seems that development of swarm forming and migratory tendency in locusts needs a genetic proclivity. This notion is supported by the fact that, when a swarm is leaving the breeding area, some individuals, which have not advanced so much towards the migratory phase, remain behind. Thus all individuals do not respond to the same extent to conditions, which tend to produce the gregarious phase. As Staal has pointed out, "It should be clearly understood that non-transient intermediates can also be produced in the laboratory under suitable conditions of intermediate density..." Another situation, supporting the notion of genetic basis for locust swarming: the advancing fronts of locust swarms have been sprayed with modern insecticides. Specially effective has been spraying from aircrafts. These operations have not only successfully checked the locust plague, they have also led to a great reduction in severity and frequency of locust swarming. It appears that the application of insecticides against moving swarms has brought about a Darwinian selection against the genes for swarming.

The devastating effect of locust migrations and swarms, and the largeness of the phenomenon may have led us to believe that this event is unique to locusts among insects. But it is not so. Similar changes in structure and behaviour in high population density may be seen in many other insects, though the changes involved may be much less marked. As pointed out by Staal, some grasshopper species, which do not show swarming, exhibit darkening of skin pigmentation and changes in bodily proportions in the state of crowding. Group stimulation is known to induce formation of well developed wings in book lice (Psocoptera) and aphids (Aphididae). The small winged grasshopper Zonocerus shows increase in wing length and short range migrations on crowding.

The legume weevils (members of the beetle family Bruchidae) of stored legumes are known to produce "normal" and "active" or "flight" phases, with several intermediate phases (Caswell, 1960; Utida, 1972; Tiwary and Verma, 1989a; George and Verma, 1994). There are many parallels between polymorphism in locust species and that in the stored legume infesting bruchids. The active bruchid individuals are, as the phase name suggests, more active than normal ones, have darker skin pigmentation and have longer wings (George and Verma, 1994). From experiments Tiwary and Verma (1989b) have inferred that deficiency of corpus allatum secretion promotes appearance of the active form in Callosobru-chus analis. Both in locusts as well as in these bruchids crowding leads to production of some migratory or specially active individuals (for bruchids see Tiwary and Verma, 1989c). Tiwary and Verma (1989b) have performed various experimental crosses between different phases of Calloso-bruchus analis, and have reached the inference that for development into the active phase a genetic proclivity is needed.

Other examples may be cited to show that the locust phenomenon is not unique to locusts. Ronkin (1978) has described a migratory phase of the potato beetle Leptinotarsa decemlineata. The beetle Chrysolina aurichalcea is flightless, and has small wings. But Suzuki has described a population of this species with flying behaviour. This population shows some significant similarity with the active phase of Callosobruchus maculatus (see Verma and Kalaichelvan, 2004).

The swarming phenomenon of locusts stands quite apart from similar other examples of production of specially active individuals on crowding among insects because of largeness of locust swarms and the huge damage they do to things of human interest. The locust problem could be largely solved through extensive investigations on their biology, through use of modern insecticides and through international cooperation. When nations with swarm producing areas shared information about the phase status of breeding locusts with the nations likely to be covered in the forthcoming locust migration, the latter could forearm themselves to control the menace. But a lot of political conflict and instability in the Middle East and in the western part of the Indian subcontinent have greatly impaired the transnational antilocust efforts. We hear at times about small swarms still being formed. The year 2004 has seen new plague locust migrations over Africa. Slackness in these multinational efforts may encourage selection in favour of the genes for swarming and migratory tendencies in locust species, and the locust problem may return.

References

Caswell, G.H. 1960. Observations on an abnormal form of Callosobruchus maculatus F..